Daniel I. Rubenstein

The Mechanisms of Filter Feeding: Some Theoretical Considerations

We enumerate the five basic mechanisms by which any biological or manmade filter can remove particles from a fluid. These mechanisms are: (1) direct interception, (2) inertial impaction, (3) gravitational deposition, (4) motile-particle deposition, and (5) electrostatic attraction. For these mechanisms we present dimensionless indexes that indicate which measurable characteristics of the filter, the particles, and the flow affect the intensity of particle capture. By comparing the magnitudes of these indexes it is possible to determine the main mechanism a filter is using to capture particles. Awareness of these mechanisms and their interrelationships will provide insights for those investigating the efficiency of various modes of filter feeding and the mechanisms of size-selective suspension feeding.

Tail size and female choice in the guppy (Poecilia reticulata)

SummaryUnder laboratory conditions, female guppies demonstrate a clear preference for males with larger tails, and this preference translates into enhanced reproductive fitness for these males. Females also prefer males with higher display rates, a behavior which appears to be linked to tail size, but which can be experimentally disassociated. This appears to be a case of female-choice sexual selection.

Social relationships and reproductive state influence leadership roles in movements of plains zebra, Equus burchellii

In animal groups, collective movements emerge from individual interactions. Biologists seek to identify how characteristics of actors in these groups, and their relationships, influence the decision-making process. We distinguished two basic factors determining leadership in group choices: identity and state. We hypothesized that identity is more important to leadership in groups with stable relationships, which permit the development of habitual roles. In groups with fluid membership, particular individuals or subgroups are less likely to emerge as consistent leaders. Instead, we predicted that movement initiation in unstable groups depends on individual state at the time of the decision. We characterized how identity and reproductive state influenced leadership patterns in the movements of plains zebra. As in many other mammals, lactation in this species significantly alters water and energy needs. We investigated leadership in tightly knit harems and loosely bonded herds of multiple harems. Harem females tended to have habitual roles in the initiation of harem movement. In herds, however, we found no consistent leaders among harems. At both levels of social organization, lactation was a key determinant of leadership. In harems, lactating females were more likely to initiate movement than nonlactating females. In turn, harems containing lactating females were more likely to lead herd movements. Thus, we conclude that social relationships and reproductive state together shape the interactions that produce group behaviours. One benefit to lactating females of leading herd movements is preferential access to scarce water. Thus, leadership roles in group decisions may have fitness consequences.

Network metrics reveal differences in social organization between two fission-fusion species, Grevy's zebra and onager

For species in which group membership frequently changes, it has been a challenge to characterize variation in individual interactions and social structure. Quantifying this variation is necessary to test hypotheses about ecological determinants of social patterns and to make predictions about how group dynamics affect the development of cooperative relationships and transmission processes. Network models have recently become popular for analyzing individual contacts within a population context. We use network metrics to compare populations of Grevy’s zebra (Equus grevyi) and onagers (Equus hemionus khur). These closely related equids, previously described as having the same social system, inhabit environments differing in the distribution of food, water, and predators. Grevy’s zebra and onagers are one example of many sets of coarsely similar fission–fusion species and populations, observed elsewhere in other ungulates, primates, and cetaceans. Our analysis of the population association networks reveals contrasts consistent with their distinctive environments. Grevy’s zebra individuals are more selective in their association choices. Grevy’s zebra form stable cliques, while onager associations are more fluid. We find evidence that females associate assortatively by reproductive state in Grevy’s zebra but not in onagers. The current approach demonstrates the utility of network metrics for identifying fine-grained variation among individuals and populations in association patterns. From our analysis, we can make testable predictions about behavioral mechanisms underlying social structure and its effects on transmission processes.

Mortality Risk of Spatial Positions in Animal Groups: the Danger of Being in the Front

We modified Hamiltons (1971) selfish herd model by introducing directional movement to the prey groups and the predators. The consequences of this modification with regards to differential predation risks are compared to Hamiltons original model (using stationary prey groups) and tested against empirical data. In model 1, we replicated Hamiltons original predator-prey system. In models 2 and 3, prey groups were mobile and predators were mobile (model 2) or stationary (model 3). Our results indicate that additional to the positive risk gradient from centre to periphery predicted by Hamiltons model for stationary groups, there might be another positive risk gradient from the rear to the front part in moving groups. Furthermore, models 2 and 3 suggest that moving groups should generally exhibit an elongated shape (longer than wide along the axis of locomotion) if risk minimisation is the only factor concerned. Also smaller inter-individual distances are predicted for front individuals than individuals elsewhere in the group. Empirical data based on the three-dimensional structure of fish shoals (using roach, Rutilus rutilus) were consistent with the above two predictions. A second experiment which involved lake chub, Semotilus atromaculatus, as prey and rock bass, Ambloplites rupestris, as predators, provided direct support for the hypothesis that individuals in front positions of groups incurred a significantly higher predation risk than fish in rear positions. Finally, we discuss the differential risks of different group positions in the context of potential foraging gains which provides the basis for a dynamic model of position preferences in group-living animals.

Herbivore-initiated interaction cascades and their modulation by productivity in an African savanna

Despite conceptual recognition that indirect effects initiated by large herbivores are likely to have profound impacts on ecological community structure and function, the existing literature on indirect effects focuses largely on the role of predators. As a result, we know neither the frequency and extent of herbivore-initiated indirect effects nor the mechanisms that regulate their strength. We examined the effects of ungulates on taxa (plants, arthropods, and an insectivorous lizard) representing several trophic levels, using a series of large, long-term, ungulate-exclusion plots that span a landscape-scale productivity gradient in an African savanna. At each of six sites, lizards, trees, and the numerically dominant order of arthropods (Coleoptera) were more abundant in the absence of ungulates. The effect of ungulates on arthropods was mediated by herbaceous vegetation cover. The effect on lizards was simultaneously mediated by both tree density (lizard microhabitat) and arthropod abundance (lizard food). The magnitudes of the experimental effects on all response variables (trees, arthropods, and lizards) were negatively correlated with two distinct measures of primary productivity. These results demonstrate strong cascading effects of ungulates, both trophic and nontrophic, and support the hypothesis that productivity regulates the strength of these effects. Hence, the strongest indirect effects (and thus, the greatest risks to ecosystem integrity after large mammals are extirpated) are likely to occur in low-productivity habitats.

Evolution, Systematics, and Phylogeography of Pleistocene Horses in the New World: A Molecular Perspective

The rich fossil record of horses has made them a classic example of evolutionary processes. However, while the overall picture of equid evolution is well known, the details are surprisingly poorly understood, especially for the later Pliocene and Pleistocene, c. 3 million to 0.01 million years (Ma) ago, and nowhere more so than in the Americas. There is no consensus on the number of equid species or even the number of lineages that existed in these continents. Likewise, the origin of the endemic South American genus Hippidion is unresolved, as is the phylogenetic position of the “stilt-legged” horses of North America. Using ancient DNA sequences, we show that, in contrast to current models based on morphology and a recent genetic study, Hippidion was phylogenetically close to the caballine (true) horses, with origins considerably more recent than the currently accepted date of c. 10 Ma. Furthermore, we show that stilt-legged horses, commonly regarded as Old World migrants related to the hemionid asses of Asia, were in fact an endemic North American lineage. Finally, our data suggest that there were fewer horse species in late Pleistocene North America than have been named on morphological grounds. Both caballine and stilt-legged lineages may each have comprised a single, wide-ranging species.

DNA metabarcoding illuminates dietary niche partitioning by African large herbivores

Significance Theory holds that sympatric large mammalian herbivores (LMH) must partition food resources to coexist, and traditional frameworks categorize LMH along a spectrum from grass-eating grazers to non–grass-eating browsers. Yet it has never been clear how finely LMH partition the enormous species diversity subsumed within these two broad plant types. By sequencing plant DNA from LMH fecal samples, we analyzed the diets of an LMH assemblage in Kenya. Diet composition was similar within species and strongly divergent across species, irrespective of feeding guild: Grazers ate similar total amounts of grass but different suites of grass species. These results suggest that species-specific plant traits may be key to understanding the dietary differences thought to underpin LMH diversity. Niche partitioning facilitates species coexistence in a world of limited resources, thereby enriching biodiversity. For decades, biologists have sought to understand how diverse assemblages of large mammalian herbivores (LMH) partition food resources. Several complementary mechanisms have been identified, including differential consumption of grasses versus nongrasses and spatiotemporal stratification in use of different parts of the same plant. However, the extent to which LMH partition food-plant species is largely unknown because comprehensive species-level identification is prohibitively difficult with traditional methods. We used DNA metabarcoding to quantify diet breadth, composition, and overlap for seven abundant LMH species (six wild, one domestic) in semiarid African savanna. These species ranged from almost-exclusive grazers to almost-exclusive browsers: Grass consumption inferred from mean sequence relative read abundance (RRA) ranged from >99% (plains zebra) to <1% (dik-dik). Grass RRA was highly correlated with isotopic estimates of % grass consumption, indicating that RRA conveys reliable quantitative information about consumption. Dietary overlap was greatest between species that were similar in body size and proportional grass consumption. Nonetheless, diet composition differed between all species—even pairs of grazers matched in size, digestive physiology, and location—and dietary similarity was sometimes greater across grazing and browsing guilds than within them. Such taxonomically fine-grained diet partitioning suggests that coarse trophic categorizations may generate misleading conclusions about competition and coexistence in LMH assemblages, and that LMH diversity may be more tightly linked to plant diversity than is currently recognized.

Sperm competition in the water strider, Gerris remigis

Abstract Sperm competition occurs in the giant water strider. In serial mating experiments using normal and sterilized males, the last male to mate was able to displace, on average, 65% of the sperm of the previous male. Success of second males was, however, highly variable but can be partially explained by relative copulation lengths of the two males. Although there was a tendency for long copulations by the first male to be followed by long copulations by the second, second males could only obtain the ‘last male’ advantage if their copulations were long relative to those of first males. A long latency period occurred between genital contact and the effective transfer of sperm as no viable eggs were produced until copulations lasted at least 15 min. Once sperm transfer began, however, virtually the entire sperm load was delivered instantaneously to the female. Females continued to lay fertile eggs for 24 days after a single copulation, and although this period increased to 30 days after two matings, the difference was not statistically significant. Although the incompleteness of sperm displacement suggests that alternative copulatory strategies among males should be common, the existence of a long latency period before sperm transfer should not only reduce this tendency, but also increase female choosiness.

Sperm competition and variation in zebra mating behavior

SummaryData are presented on the breeding behavior of two zebra species to test whether intra- and interspecific variation in male reproductive behavior and physiology are correlated with differences in female promiscuity. In one species, plains zebra (Equus burchelli) females live in closed membership single male groups and mate monandrously. In the other species, the Grevys zebra (E. grevyi) females live in groups whose membership is much more temporary. Typically, associations with individual males are brief and mating is polyandrous. However, some females - those having just given birth — reside with one male for long periods, mating monandrously. These differences in female mating behavior generate variability in the potential for sperm competition. We show that behavioral differences in male investment in reproductive activities correlate with the potential for sperm competition. When mating with promiscuous mares, Grevys zebra stallions made a greater investment in reproductive behavior (calling, mounting, ejaculations) than did stallions of either species when mating with monandrous females. The evolution of large testes size in the Grevys zebra, when compared to the congeneric plains zebra, horse, and mountain zebra, allows for this increased investment.